The benefit under 35 USC 119 is claimed of German application No. 197 48 188.4 filed Oct. 31, 1997.
The present invention relates to a device for reading from or writing to optical recording media, which have a data track having markings arranged both centred with regard to the said data track and offset with respect to the said data track.
A device of this type is disclosed in EP-A-0 457 536. This device is suitable for reading an optical recording medium which has a data track having a first area which is provided for markings arranged centred with respect to the centre of the data track. These markings are often referred to as pits as well. The data track has, moreover, further areas in which markings are provided which are arranged laterally offset with regard to the centre of the data track. The device has a beam generation means for generating a beam which is directed onto the data track and is guided by a tracking means along the data track. A detection means converts the beam which comes from the recording medium and is modulated by the latter into detector signals which are fed to a track error detector for the purpose of determining a track error signal, and to a signal shaping means for the purpose of forming a shaped data signal. The shaped data signal is in this case subjected to a sample and hold operation in each case at the beginning of one of the offset areas. The difference between two held signals is formed, integrated and added to the track error signal, determined by the track error detector, as correction signal. The corrected track error signal is fed to the tracking means. It may be offered as a disadvantage of this known device that the determination of the correction signal is highly time-critical since the value of the shaped signal is sampled and then held only in accordance with a brief pulse in each case. This leads to an inaccurate result since the data signal rises and falls to a great extent, as a result of which even small temporal inaccuracies in the course of sampling the value to be held have a major influence on the held value.
The object of the present invention is to propose a device of the generic type in which an accurate correction signal for the track error signal can be obtained without a high outlay, and also a method for generating a corresponding correction signal.
This object is achieved by the fact that the signal shaping means has an averaging unit, the output of the averaging unit being connected to an input of the correction signal generation means. This has the advantage that higher accuracy can be achieved since an averaged value is fed to the correction signal generation means, which value has fewer large fluctuations. A further advantage is that it is possible to identify even small deviations in the average value which are not conspicuous on observation of the shaped data signal. Even such small deviations are taken into account in the device according to the invention, which contributes to the high accuracy of the correction signal.
The averaging unit is advantageously part of a so-called data slicer, which generates a modulated signal from the signal modulated by the recording medium by comparison with the comparison signal supplied by the averaging unit, which modulated signal is the input signal of the averaging unit. This has the advantage that the data slicer, as an element of the device which is present in any case, is additionally utilized for correction signal determination, thereby obtaining a reduced outlay.
The invention provides for the averaging unit having at least two comparison signal generation paths, which can be activated by a switching means in dependence on the scanned area of the data track. This has the advantage that an average value or comparison value which is not influenced during the scanning of the other areas of the data track, and, consequently, during renewed scanning of a corresponding area, has with high probability the optimum value for this area, is formed for each area of the data track. Thus, the use of different comparison signal generation paths also increases the quality of the shaped data signal since negative influences which may be caused by the other areas are avoided.
It is advantageous for the correction signal generation means also to have at least two signal processing paths, whose inputs can be connected by the switching means to the input of the correction signal generation means. Here, too, the advantage is that the average or comparison value is optimized to an area of the data track and not influenced by the other areas, as a result of which fluctuations in the average value, which is often referred to as slice level as well, in the event of a change from one area to another do not occur. The quality of the correction signal generated is thereby increased.
The invention provides for a holding element or an integration element to be assigned to a signal processing path. This has the advantage that a fast reaction is enabled via the proportionally acting holding element, while overshooting given the possible occurrence of defective pits is avoided by means of an integration element. It is particularly advantageous to provide a plurality of paths, of which one portion is finished with a proportional characteristic and one portion is finished with an integrating characteristic, the signals of which are subsequently combined in order to exploit the advantages of both characteristics.
The invention furthermore provides for the output of a first and of a second signal processing path to be connected to the input of a difference forming unit. This has the advantage that the relative central position of the areas of the data track which are assigned to these signal processing paths is defined. If the areas are removed to the same extent, but in different directions, from the central position of the data track, then the relative central position should correspond to the value zero. If it deviates from this, then correction is necessary. If the distances between the respective areas and the centre of the data track differ, then weighted difference formation is advantageously provided, which takes account of this asymmetry.
It is advantageous for the outputs of a plurality of difference forming units to be summed. This has the advantage that an optimum correction signal is obtained by the combination of a plurality of signal processing paths, for example having a proportional and an integrative characteristic.
The invention furthermore provides for the correction signal generation means to have at least one amplification element having a variable gain factor. This has the advantage that the complete correction signal can be adapted by amplification to different device properties or different conditions. Different operating conditions are governed for example by the type of recording medium currently being used, this being expressed in the reflectivity, for example.
According to an alternative configuration of the invention, it is provided that the correction signal generation means has at least two signal processing paths, whose inputs can be connected by a switching means to at least one output of the detection means in accordance with the respectively scanned area of the data track. This has the advantage that the correction signal is generated even if the averaging unit should not generate a correct output value. For example, the signal of a single detector element is present at the output of the detection means, but provision is preferably made for using a combination of the signals of a plurality of detector elements, such as the summation signal of all the detector elements, for example. The signal processing paths preferably evaluate the amplitude of the signal respectively present and hold their output value as long as no input signal is present.
The method according to the invention consists in the track error correction signal being formed from a comparison signal, with which is compared a detector signal of the detection means for the purpose of forming a shaped data signal. This has the advantage that a signal which is present in any case is advantageously utilized for the purpose-of forming a correction signal. It is thus not necessary to form an additional signal for correction signal generation.
The invention provides for different comparison signals, which are used to generate the track error correction signal, to be provided for the scanning of different areas of the data track. This has the advantage that the quality of the shaped data signal is increased since errors which may occur during a transition between the individual areas of the data track are avoided by this means.
It is advantageous to make use exclusively of comparison signals which correspond to one of the further areas of the data track, that is to say markings arranged offset with respect to the centre of the data track. This has the advantage that the correction signal formation is performed particularly effectively since the said areas have a relatively large deviation which is relatively simple to detect.
Further advantages of the invention will also emerge from the following description of advantageous exemplary embodiments of the present invention. It is understood here that the invention is not restricted to the exemplary embodiments specified.